Sjögren's syndrome (SS) is a chronic autoimmune disorder of unknown cause. Haneji et al identified a 120 kDa fragment of the ubiquitous cytoskeletal protein α‐fodrin as an autoantigen in an NFS/sld mouse model of human SS.1 More recently, the sensitivity and specificity of α‐fodrin antibodies in SS in several studies suggested that it might be of value in the diagnosis of SS.1,2,3 In the present study, a conserved motif, rather than the whole protein, in α‐fodrin was used as antigenic peptide to define further the role of anti‐α‐fodrin antibodies in SS. This is the first study so far to evaluate the prevalence of anti‐α‐fodrin peptide antibodies (α‐FPA) in a cohort of Chinese patients with SS.
Serum samples from 89 patients with SS (85 female, 4 male; 74 with primary SS (pSS) and 15 with secondary SS (sSS) associated with rheumatoid arthritis (RA) or systemic lupus erythematosus (SLE)) were examined. Their mean (SD) age was 54 (6.5) years, and the mean (SD) disease duration was 6.6 (3.8) years. A few patients were receiving steroids or immunosuppressive drugs. Serum samples from 41 patients with SLE, 44 with RA, and 59 healthy subjects were analysed as controls. All the patients with SS were carefully evaluated based on the European‐American criteria.4 Patients with RA and SLE fulfilled the American College of Rheumatology (ACR) criteria for classification. A sequence of the α‐fodrin derived peptide (RQKLEDSYRFQFFQRDAEEL)1 was synthesised by a solid phase technique on an Applied Biosytems Peptide Synthesizer. Synthesised α‐fodrin peptide was used as substrate in an enzyme linked immunosorbent assay (ELISA) to detect IgG α‐FPA in serum samples of the patients and the controls.
The IgG α‐FPA were present in 54/74 (73%) patients with primary SS, 6/15 (40%) with secondary SS, 18/41 (20%) with SLE, 13/44 (30%) with RA, and 2/59 (3%) normal blood donors (table 1). The prevalence of α‐FPA in primary and secondary SS was significantly higher than in SLE, RA, and normal controls (p<0.01 in each case).
Table 1 Prevalence of α‐FPA in patients with SS, RA, and SLE.
| Patients | α‐FPA | ||
|---|---|---|---|
| (n) | Positive | OD Values | |
| No (%) | Mean (SD) | ||
| SS | 89 | 60 (67) | 0.30 (0.08) |
| Primary SS | 74 | 54 (73) | 0.29 (0.08) |
| Secondary SS | 15 | 6 (40) | 0.34 (0.09) |
| SLE | 41 | 8 (20)† | 0.23 (0.09)* |
| RA | 44 | 13 (30) | 0.28 (0.09) |
| Healthy controls | 59 | 2 (3)† | 0.17 (0.04)* |
*p<0.05; †p<0.01 in comparison with the group with SS.
The sensitivity and specificity of α‐FPA in SS were 73% and 88.7%, respectively. The sensitivity of α‐FPA in pSS was much higher than anti‐SSA, anti‐SSB, and antinuclear antibodies (ANA; table 2). The specificity of α‐FPA (81.8%) was similar to that of anti‐SSB (85.0%), and was higher than that of anti‐SSA (64.7%) and ANA (42.6%) in the diagnosis of pSS. A high prevalence of α‐FPA was found in patients with SS who lacked anti‐SSA, anti‐SSB, and ANA. The presence of α‐FPA was closely associated with extraglandular manifestations, such as pulmonary fibrosis, renal failure, and vasculitis.
Table 2 Sensitivity and specificity of α‐FPA in 74 patients with primary SS.
| Antibodies | Positive | Sensitivity | Specificity |
|---|---|---|---|
| (n) | (%) | (%) | |
| α‐FPA | 54 | 73.0* | 81.8† |
| Anti‐SSA | 38 | 51.4 | 64.7 |
| Anti‐SSB | 9 | 12.2 | 85.0† |
| ANA | 47 | 63.5 | 42.6 |
*p<0.01, compared with anti‐SSA, anti‐SSB, and ANA; †p<0.05, compared with anti‐SSA and ANA.
Previous studies have shown that the antigenic epitope of α‐fodrin is within the amino terminal region and masked in the entire amino acid sequence. To analyse the significance of α‐fodrin antibodies in SS, a conserved α‐fodrin peptide was used as the antigen in this study. The results suggest that α‐FPA are a serological variable in SS, and are more sensitive than ANA, anti‐SSA, and anti‐SSB. α‐FPA appear in the early stage of SS, indicating that these antibodies are valuable for early diagnosis of the disease. We have also shown that α‐FPA are associated with pulmonary interstitial fibrosis, vasculitis, and renal disease, which suggests that α‐FPA has a pathogenic role in SS.
The 120 kDa α‐fodrin is abundant in the epithelial cells of the human salivary gland and can stimulate peripheral blood T cells in patients with SS.5 It is suggested that α‐fodrin may undergo a redistribution and relocate to the cell membrane of apoptotic cells in human SS.6 The autoantigen on the surface of apoptotic cells is involved in antigen presentation processing and induction of autoantibodies. We speculate that cleavage and altered distribution of α‐fodrin in glandular epithelial cells may impair the secretory function and perpetuate an autoimmune response to α‐fodrin, leading to production of autoantibodies and glandular destruction in SS.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (39970697) and National Outstanding Youth Grant of China (30025040).
Footnotes
Competing interest: None.
References
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